Means for converting sound into electrical impulses



H. C. SNOW April 5, 1932.

MEANS FOR CONVERTING SOUND INTO ELECTRICAL IMPULSES Filed March 9, 1951 \nvenTor. Howard C. Snow WW WkW ATT ys.

Fig.2.

Patented Apr. 5, 1932 PATENT OFFICE HOWARD O. SNOW, OF LYNN, MASSACHUSETTS MEANS FOR CONVERTING SOUBTD INTO ELECTRICAL IMPULSES Application filed. March 9, 1931.

This invention relates to electrical apparatus for converting sound vibrations in the electrical impulses, and particularly to the type of apparatus commonly called a microphone in which the movement of a diaphragm relatively to a magnet functions to generate electrical impulses in a coil or conductor of any character associated with the magnetic field produced by the magnet.

The general object of the invention is to eliminate such vibrations of the diaphragm as will interfere with the production of electrical impulses which correspond accurately with the frequency and magnitude of the original sound vibrations, and particularly to eliminate the harmonic vibrations of the diaphragm itself.

A further object of the invention is to pro vide a microphone of increased sensitiveness in which the diaphragm is movable bodily.

A further object of the invention is to provide a microphone in which a diaphragm hav ing low inertia is suspended upon a plurality of filaments under tension which are substantially longer than the diameter of the diaphragm.

A further object of the invention is to provide a microphone in which damping of the diaphragm may be controlled readily by manual means.

A further object of the invention is to provide a microphone in which the sound vibrations are conducted to an enclosed chamber in which the diaphragm moves bodily as a piston.

These and other objects and features will more fully appear from the folowing description in connection with the accompanying drawings, and will be particularly pointed out in the claims.

The apparatus comprises essentially a means for producing an intense magnetic field. Such a means is preferably an electromagnet having a core system including a ring-shaped air gap in which is received a coil of wire which may be constructed to be selfsupporting, or the wire forming the coil may be wound upon a hollow cylindrical form. In the latter case the coil and its form are secured to a diaphragm, preferably of material Serial No. 521,199.

which is light in weight and nonmagnetic. The coil and the diaphragm constitue the moving element of the device, and may be termed the armature. The armature is suspended in such position relatively to the electro-magnet that the coil may be moved axially within the air gap. The means of supporting the armature consists of two or more filaments under tension, which may be of fiat ribbon, or of round metallic wire. The filaments, however, may be of any suitable material. The ends of the filaments are secured in a rigid frame which in turn is secured to the magnet.

To increase the effectiveness of the apparatus a mouth piece can be provided to direct the sound to a closed chamber surrounding the diaphragm. The purpose of the chamber surrounding the diaphragm will be set forth hereinafter.

In the drawings:

Fig. l is a plan view of the apparatus looking into the mouth piece.

Fig. 2 is a cross sectional view on line 22 Fig. 1.

Fig. 3 is a detail View showing the construction of the coil and diaphragm and their relation to the poles of the magnet.

In order to provide a better understanding of the invention a particular embodiment of the apparatus will be described. In order to obtain the maximum effectiveness of the device a magnetic field of high intensity must be provided. Such a field is provided by means of an electro-magnet designated generally at 1. A desirable form for the magnet is illustrated herein in which a coil of wire 2 consisting of a number of turns, is adapted to be connected to a source of electric current. The coil is wound upon a pole piece 8 which extends axially through the coil, and projects beyond the upper limit thereof, and is secured at its lower end to a heavy casing of magnetic material 4. The pole piece 8 constitutes one pole of the magnet while the casing 4 forms the other pole. In the upper wall of the easing 4 a cylindrical opening 5 is formed somewhat larger than the end of the pole piece 3 through which the pole piece extends.

The circular end of the pole piece 3 is situated in concentric relation to the opening 5 in the casing thereby forming a ring-shaped opening. A coil of wire 6 which is called commonly the voice coil is received within the ring-shaped opening between the pole pieces.

The turns of the voice coil 6 may be made self-supporting by bonding with cement or other material or preferably are wound upon a supporting-tube 7. The coil and tube 7 are of such dimension that a slight clearance exists between them and the poles of a magnet. While this is a desirable form of magnetic system, other forms of magnets may be inployed however. The coil vith its supporting tube is secured to and supported by the diaphragm 8 which is of non-magnetic material and of light weight. The coil 6 and the diaphragm 8 constitute the moving element or armature of the apparatus and are supported by means of two or more filaments which are desirably sections of metal wire 9. Any suitable material may be employed however as a supporting filament. Iron wire has been found to be ver effective since the slight attraction of the adjacent magnet serves to damp out extraneous undesirable vibrations set up in the filaments themselves. This magnetic attraction for the iron wire also aids in maintaining a uniform tension therein. The length of the wires and a method of supporting them at their ends may be varied to suit the particular requirements of the use to which they are to be put. As herein shown the wires 9 are supported adjustably in a frame 10 secured to the upper face of the easing 4 of the magnet, and are stretched between eye bolts 11 which extend through the ends of the frame, and are provided with nuts 12 by means of which the required tension may be applied to the wires. Any suitable means may be employed to secure the diaphragm 8 to the wires 9. As herein illustrated the wires are cemented or otherwise cured to the diapragm at one or more points,

at 13. The method of supporting the diaphragm permits it to move bodily with the motion of the Wires without bending or distorting its normally flat surface.

The frame 10 is secured upon the magnet by means of screws 14 which extend through oversized apertures 15 therein. The clearance thus provided between the screws 14: and the apertures 15 permits the frame to be justed within small limits to center the coil 6 properly within the aperture 5 in the magnot To aid in making this adjustment a series of apertures 16 formed in the diaphragm at the proper points to permit the use of feelers in making this adjustnr and to allow rough adjustment by eye. In order to prevent sympathetic vibration between the wires 9 and to aid in damping out free vibration in the floating portions of the wires, the length of the wires should be un equal as shown in Fig. 1.

In addition to the novel method of supporting the diaphragm novel means are provided. also to increase further the effectiveand efficiency of the apparatus. To increase the response of the diaphragm 8 to vibrations set up by sound, it is enclosed in a casing 17 secured to the upper face of the magnet, the side walls of which are in close proximity to the edge of the diaphragm, but do not contact therewith. Slots 18 are formed in the side walls of the casing 17 through which the supporting wires 9 pass. The slots 18 are of sufficient length to allow the maximum amplitude of vibration of the diaphragm without contacting with the easin g. A mouth piece 19 is provided also which removably connected to the casing 17 and acts to direct the sound waves into the Cl'l:l11'l ber 20 within the casing. As a result of this co; gi'zruction the diaphragm functions as a pm on moving within the casing 17 which. acts e a-operating cylinder; thus practically 2 ta]v energy of the sound vibration entering the chamber is utilized in moving the diaphragm. Inasmuch as the clearance between the tube 7 of the armature assembly and the pole piece 3 is very small, means are provided to prevent serious interference in the motion of the diaphragm by the air trapped within the space between the end of the pole piece and the diaphragm. This may be done in any convenient way. A desirable construction is illustrated wherein the pole piece 3 is constructed as a hollow cylinder having longitudinal aperture 21 extending throughout its length thereby permitting free passage of air currents set up by the lower face of the diaphragm. If desired, a valve 22 may be introduced in the aperture 21 thereby to ermit the damping effect of the trapped air below the diaphragm to be normally controlled as it very often becomes desirable to introduce a certain amount of damping to overcome undesirable vibration.

In order to suppress excess vibration which may occur in wires 9 due to their natural period of vibration or to other influence, it is desirable, but not essential to provide a plurality of pads 23 of felt or other resilient material placed in light contact with the wires 9 at a point which may be best determined by experimentation. As herein shown four such pads are provided placed at points near the ends of the wires and supported by cross bar members 24 secured in the frame 10.

The ends of the voice coil may be connected to the external circuit in any suitable way. A desirable method is to connect the ends of the coil 6 to the wires 9. This is possible in the embodiment shown for the reason that the diaphragm is constructed of insulating material. The wires 9 are utilized therefore to carry the current generated by the coil to the ill eye bolts 11 Which are in turn connected to the external circuit by means of the wires 25. Electrical insulation is provided at this point by insulating the eye bolts from the frame 10 by means of sleeves 26 of insulating material enclosing the shanks of the eye bolts, and inserted in the ends of the frame 10. The ends 27 of the coil 2 of the magnet are connected to a source of electrical current such as a battery 28.

lVhen the apparatus is spoken into or permitted to intercept sound waves from any source, the diaphragm with its attached coil is caused to move bodily toward and from the pole piece 3. In doing so all turns of wire on the coil 6 are caused to cut the lines of force traversing the air gap between the ends of the poles of the magnet. A current thus is generated in the coil corresponding to the movements of the diaphragm in frequency and magnitude. These impulses of current flow into the external circuit where they may be amplified by means of a vacuum tube amplifying device or they may be conducted directly to any desired type of apparatus capable of recording the impulses. They may be conducted for instance to a sound-reproducing device.

The common form of microphone wherein a stretched diaphragm is supported rigidly at its periphery is not an accurate means of interpreting sound waves since the undesirable harmonics set up within the diaphragm act to damp out and generally modify adversely the vibrations of the diaphragm. The present invention as compared to such a device is acoustically efficient and functions accurately to interpret sound vibrations. These favorable characteristics may be attributed to the fact that the diaphragm has a very low inertia, and does not function by distortion of the material within its own area, but by a bodily movement. Due to the fact that the diaphragm is secured to its co-opcrating coil there can be little or no vibration set up Within the diaphragm. Whatever vibration may be present will be so small as to have no effect upon the character of the impulses generated by the apparatus.

Another valuable feature of the invention is the ruggedness of the apparatus which permits it to be carried about without fear of damage. The ruggedness of the apparatus is I due chiefly to the fact that its elements may be of reasonably large dimensions and adjust ments are not of an extremely delicate character as are those in certain types of microphones commonly used. Another valuable feature Which results from the sturdy character of the apparatus is the fact that it is comparatively insensitive to moderate changes in temperature.

The novel method of suspending the arma- 7, ture of the device in the manner above described results in preventing the so-called blasting and ringing sounds which are a common characteristic of most microphones, and for this reason the microphone may be carried about and receive considerable mechanical jarring without causing disagreeable sounds from the reproducin equipment to which it may be connected. The invention is also electrically eilicient and capable of accurately interpreting a wide range of sound frequencies, and is especialy capable of handling deep base sounds, since its sensitiveness to sound of low frequency is directly proportional to sound of medium or high frequency, and its maximum amplitude of vibration as compared to microphones at present in use is relatively great.

While the apparatus above described includes an electro-magnetic means for generating the varied currents in the external circuit any other suitable means for exciting this circuit may be employed such, for instance, as the form of microphone commonly known as the carbon button microphone, or the condenser type of microphone may be employed in connection with the novel means of supporting the diaphragm.

Having thus described the invention, what is claimed as new, and desired to be secured by Letters Patent, is:

1. A microphone comprising an electromagnetic system having a hollow cylindrical pole piece constituting one pole of themagnet and co-operating with the other pole of the magnet to form a cylindrical air gap therebet-ween, a coil of wire situated within the air gap, a diaphragm secured to the coil,

chamber surrounding the diaphragm provided with an opening to permit sound waves to act upon the diaphragm and having its side walls situated in close proximity to but not engaging the periphery of the diaphragm, a frame, a plurality of filaments under tension supporting said diaphragm and coil in a manner to permit bodily movement thereof as a piston within said chamber, and a manually-operable valve within said hollow pole piece whereby the passage of air currents within the pole piece set up by the motion of the diaphragm may be controlled to damp the diaphragm.

2. A microphone comprising a diaphragm, means electrically connected to an external circuit operable by movement of the diaphragm to set up varying currents within the circuit, a frame, a plurality of filaments of different lengths substantially longer than the diameter of the diaphragm and fixed under tension within said frame supporting said diaphragm in a manner to permit bodily movement thereof in consonance with sound waves acting upon the diaphragm, means upon the frame to adjust the tension of said filaments and a plurality of pads of resilient material engaging the floating portions of said filaments whereby undesirable vibrations set up Within the filaments will be damped.

3. A microphone comprising a magnetic system having a hollow cylindrical pole piece constituting one pole of said system and cooperating with the other pole of the said system to form a cylindrical air gap therebetween, a coil of wire situated within said air gap, a diaphragm secured to said coil, a frame, a plurality of filaments under tension secured at their ends to said frame acting to support said diaphragm and coil in a manner to permit bodily movement thereof, such movement having the same amplitude throughout the area of the diaphragm and manually-operable means for controlling the passage ofair currents within the pole piece set up by the motion of the diaphragm thereby to dampen the movements of the diaphragm.

In testimony whereof, I have signed my name to this specification.

HOWARD G. SNOW. 

